It is well known that catalytically or thermically operating exhaust gas cleaning arrangements in order that they could attain a high efficiency in their cleaning activity, are operated at high temperatures. It is, however, also known that a very high temperature may lead to an untimely destruction of the cleaning system. Extremely high and inadmissible temperatures in the exhaust gas usually occur when there is a failure in the ignition system, or there is an insufficient combustion of the gases within the combustion space or in the event there is an improper mixture formation within the combustion space and thereby the combustion takes partly place in the exhaust gas cleaning system instead of within the combustion space.
Attempts have been made to cure the abovementioned disadvantages and defects and one such attempt is represented by German Utility Model No. 1,917,133 - 46 c 6-6/61 which describes an arrangement having in its catalyzator a temperature sensor provided which, in the event very high and undesirable temperatures are present in the bed of the catalyzator will operate a valve which bypasses the exhaust gases about the catalyzator bed. As a result a destruction of the catalyzator bed by the extremely high exhaust gas temperature can be prevented, however, this is done only by disconnecting the exhaust gas cleaning system and thereby emitting the uncleaned exhaust gas into the atmosphere.
From German laid open application No. 1,921,024 a device for cleaning the exhaust gases is known as using a separately heated thermal reactor in which, in the event undesirably high temperatures are developed in the reactor, the fuel for its heating is disconnected. This means that the reactor in certain circumstances becomes inoperative and, the danger that it can be overheated by operating defects such as failure of the ignition etc., as mentioned above, is still present. In this device after the heating fuel to the reactor has been disconnected a chemical transformation of the exhaust gas components takes place at a positive heat generation.